1) Field of the Invention
The present invention relates to a process for producing the polyester polyethylene terephthalate (PET) from a lower dialkyl ester of a dicarboxylic acid and a glycol using a specific catalyst system which improves not only the ester interchange time but also the polymerization time. In particular, the catalyst system comprises manganese, lithium, cobalt and antimony. More specifically, manganese and lithium are used as catalysts for the ester interchange while cobalt and antimony are used as catalysts for the polycondensation stage.
2) Prior Art
The catalyst system of the present invention is specific for manufacturing PET from the starting materials of a lower dialkyl ester of a dicarboxylic acid (LDE) such as dimethyl terephthalate (DMT) and glycol, such as ethylene glycol. More specifically, DMT and ethylene glycol are typically reacted in the presence of a catalyst (manganese) at atmospheric pressure and at a temperature of from about 180.degree. C. to 230.degree. C. In the presence of a suitable catalyst, these components undergo ester interchange to yield bis (2-hydroxyethyl) terephthalate or "monomer" and methanol. The reaction which is conventionally done with 1 mole of DMT and 2 to 2.4 moles of ethylene glycol is reversible and is carried to completion by removing the methanol formed. During the ester interchange, the monomer is the substantial majority product (not considering the methanol) along with small amounts of low molecular weight polymers and unreacted components.
The monomer is then polymerized where the temperature is raised to about 280.degree. C. to 310.degree. C. and the pressure is reduced to below 1 mm of mercury vacuum and in the presence of a suitable polymerization catalyst (antimony). From this reaction, polyethylene terephthalate (PET) and ethylene glycol are formed. Because the reaction is reversible, the glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester. This known process is described in U.S. Pat. No. 4,501,878 to Adams.
Manganese is the preferred catalyst for ester interchange reactions, but the amount of manganese employed must be strictly controlled. The presence of too litle manganese during the ester interchange reaction results in very long reaction times, while the presence of too much manganese results in unwanted side products during the polycondensation reaction (thus lowering the yield of monomer), and unacceptable degradation of the polymer resulting in poor color (thus lowering the quality). The exact range of manganese which proves to be the most desirable must generally be determined through trial and error because many factors affect the reactivity of the manganese. For example, reaction temperature, reaction pressure, the degree of mixing during reaction, the purity of the raw materials, the presence of other additives, etc., all affect the effectiveness of manganese.
In prior art processes, manganese was employed to obtain suitable ester interchange reaction times, but the manganese had to be sequestered after ester interchange or during polycondensation by a polyvalent phosphorous compound to aid in reducing the discoloration and unwanted side products. Generally, prior art processes employed about 50 ppm to 150 ppm manganese based on the expected yield of the polymer, as the ester interchange catalyst. Using more than about 150 ppm manganese resulted in polymer degradation even if phosphorous was employed in excess to sequester the manganese. It is believed that this occurred because the phosphorous was incapable of complexing with the manganese to the degree necessary to prevent discoloration.
U.S. Pat. No. 3,709,859 to Hrach et al discloses a multi-component catalyst system for producing polyester. Among the many catalysts mentioned are lithium, cobalt, manganese and antimony. Although these catalysts are set forth in the background portion of the patent, the patent claims a catalyst system comprising antimony, lead, and calcium, and additionally strontium or barium. Hrach et al also teach the necessity of employing pentavalent phosphorous compounds as stabilizers in order to prevent the formation of discolored polyester.
U.S. Pat. No. 3,657,180 to Cohn discloses a process for making polyester resin in which lithium or a divalent metal compound are employed as catalyst during the production. The specification states that manganese may be one of the divalent metallic compounds which can be employed. The order of mixing the various raw materials and the addition of the compounds to the process described in this invention is stated to be critical. The process is carried out by reacting DMT and ethylene glycol in the presence of a lithium salt under ester interchange conditions followed by the addition of manganese. The process also includes using manganese as a catalyst with lithium being added after the ester interchange reaction. In either case, the second metal is always added after ester interchange, and thus is not used as a catalyst. Moreover, the second metal is always added in a non-catalytic amount. The second metal is added to provide slip for polyester film and the amount added is several factors larger than catalytic amounts.
British Patent 1,417,738 to Barkey et al discloses a process for manufacturing polyester in which a preferred ester interchange catalysts may include zinc, manganese, cobalt, and lithium, among others. Preferred polycondensation catalysts include antimony compounds. This reference, however, claims other catalyst compounds and mentions the above catalyst only as background information.
Various patents assigned to Eastman Kodak Company (British Patents 1,417,738, and 1,522,656; U.S. Pat. Nos. 3,907,754, 3,962,189, and 4,010,145) disclose a broad variety of catalyst systems, including a manganese, cobalt, lithium and titanium combination and a manganese, titanium, cobalt and antimony catalyst system, with phosphorous being used in each of these systems as a sequestering agent. Each of these catalysts was added at the beginning of ester interchange. Although these catalyst systems would generally reduce the overall time required to process the raw materials into polyester, because the ester interchange time was substantially improved; the polycondensation time was not substantially improved.
Improvements which reduce the ester interchange time, but not the polycondensation time, for example, are not particularly advantageous especially where different reactor vessels are employed for the ester interchange process and the polycondensation process. When different reactor vessels are employed, a reduction in only the ester interchange time, for example, does not necessarily reduce the total process time, because the total process is only as fast at the slowest stage in the process. Therefore, a reduction in time for one of the two stages may not improve the overall existing process. In such a situation, additional reactor vessels could be purchased for the slowest stage to improve the total process time, but this is an expensive solution.
There remains a need to develop a catalyst system and process which will reduce not only the ester interchange reaction time but also the polycondensation reaction time so that the totality of processing time is substantially reduced.
It is a further aim or aspect of the present invention to not only quickly produce a polyester, namely PET, from raw materials, but produce a polyester which has acceptable clarity, IV and color properties.